In regard to the conventional ink supply units and inkjet printing apparatuses, Patent Literature 1 discloses an ink supply device for inkjet printers characterized by the following technical features; ink tanks containing different color inks are located distantly from color ink nozzle units, ink buffers disposed in the color ink nozzle units are connected to the ink tanks with ink supply tubes and ink replenishment tubes, the ink supply tubes each have an energizing means, and the ink replenishment tubes each have a tube opening/closing means. This ink supply device is operable to select any one of initial ink supply to the color ink nozzle units, deairing, and ink replenishment during priming and printing by choosing to activate or deactivate the energizing means, and choosing to open or close the tube opening/closing means. The ink tanks of this ink supply device containing different color inks each have an upper part configured to have an ink cassette be attachable and detachable to and from the upper part via a liquid level conserving means.